The single borehole from which deviated boreholes are drilled in the present invention is in general a vertical borehole. In order to reach widely spread targets one or more deviated borehole branches are drilled from the single borehole. A kick-off device is often used to initiate drilling of a deviated borehole from the single borehole. The kick-off device may be, for example, a device in the form of a steel wedge or whipstock arranged at the point in the single well where the deviated wellbores are to be initiated. On continuing drilling, the drill bit follows the wedge and a deviated borehole branch is drilled. A description of this method may be found in The Petroleum Handbook, 6th Edition, Elsevier, 1983, pages 135-136.
When it is required to drill two of opposite borehole branches the second borehole branch can be drilled using a whipstock oriented in a direction opposite to the direction of the first borehole branch.
A kick-off device used to drill more than one deviated borehole branch is disclosed in U.S. Pat. No. 4,573,541. This device comprises a tubular housing and three guide tubes arranged in the housing. Two of the guides are curved away from the central longitudinal axis of the housing. The deviated borehole branch is drilled by arranging the device in the lower end of the cased single borehole, guiding the drill bit into one of the two curved guide tubes and drilling.
The kick-off device of U.S. Pat. No. 4,573,541 requires that the diameter of the guide tubes be smaller than half the diameter of housing. The curved borehole branches must therefore have a diameter that is much smaller than the diameter of the original borehole from which they are drilled.
It is therefore an object of the present invention to provide a kick-off device that allows drilling a pair of deviated borehole branches from a central wellbore, extending in opposite directions from the central wellbore, wherein the maximum diameter of the branched borehole section is limited by the diameter of the drill bit that can pass through the cased single borehole. It is a further object to provide such a method wherein the pair of deviated borehole branches may extend from about the same point in the central wellbore.